I think it would be best to avoid the CR landmine. What is a CR 4 may kill a party that doesn't use tactics or be a cake walk to others. It depends a lot on party makeup, how they made the characters, party formation, etc. Too many variables to accurately try to pin down a CR.

In a tabletop game I held off the party of Lvl 4 to 5 characters with about 4 lvl 1 goblins that were using total cover and alchemist fire. One single fireball down the center of goblins would have ended the battle quickly. Like I said, it is very situational.